The present invention relates to the art of calibrating RF pulses. It finds particular application in conjunction with magnetic resonance imaging and will be described with particular reference thereto. However, it is to be appreciated that the invention has other applications such as magnetic resonance spectroscopy.
In magnetic resonance imaging, radio frequency pulses are applied to tip the magnetization vector of resonating nuclei by selected tip angles. If the radio frequency pulses produce a tip angle which differs from a selected tip angle, the difference or error will cause errors in the results of the magnetic resonance procedure.
In magnetic resonance imaging, each patient changes the loading on the coil. The change in load, as well as differences in patient geometry, changes the power level of the RF pulse required to produce a selected tip angle. In each patient, a given RF pulse power produces a different tip angle in accordance with the loading differences from patient to patient. To eliminate this source of error, the RF pulses are conventionally recalibrated for each patient.
Various calibration techniques may be implemented. In one technique, a magnetic resonance sequence is implemented to elicit a magnetic resonance signal which is sensitive to the RF tip angle. The variations in the magnetic resonance signal are correlated and analyzed to give an indication of the RF energy required to yield the selected tip angle in vivo.
The RF calibration may depend on the formation of a conventional spin echo signal. An overall RF scale factor is adjusted in order to maximize the spin echo magnitude. More specifically, a sequence is chosen which produces a spin echo signal. A maximum RF value is determined from the sequence, taking into account SAR constraints. The RF scale value is incremented from zero to its maximum in a selected plurality of equal increments. At each incremental value, five repetitions of the RF sequence are produced. The spin echo signal from the last two repetitions are averaged and the peak magnitude is recorded. This procedure is repeated until a maximum peak magnitude is identified. A new RF scale region is defined with the lower limit being the incremental point just prior to the maximum and the upper limit being the incremental point to the other side of the maximum. The RF scale value is incremented through the new region from its lower limit to its upper limit in the preselected plurality of smaller increments. Peak magnitude data is recorded as described above for each incremental point, independent of the position of the maximum. The position of the maximum peak magnitude is determined by data interpolation and another new RF scale range is determined. Incrementing the RF scale value and recording the peak magnitude for each incremental point is repeated for the new RF scale value range. The RF scale value which results in the maximum peak magnitude of the latest RF scale value range is determined by data interpolation. Appropriate 90.degree. and 180.degree. tip angles are determined from this RF scale value.
Although this technique sets the 90.degree. and 180.degree. tip angles, it is time consuming and not necessarily completely accurate. Typically, about two minutes are required for this sequence to be computer implemented.
In another technique, the RF calibration procedure is much faster, but at the cost of very low accuracy around the 180.degree. tip angle. Three identical RF pulses of an as yet unknown tip angle .alpha. are applied in the presence of a constant gradient. The three pulses produce a number of echoes including spin echoes and a stimulated echo. Each echo had a specific dependence on the intensity of the tip angle .alpha., the timing parameters, and the relaxation parameters. For the combination of the stimulated echo and the first spin echo, the ratio of the intensity varies strongly with the angle .alpha., more specifically 2cos.sup.2 (.alpha./2). Because the intensity of the two echoes is readily measurable, the angle .alpha. can be readily calculated. By adjusting the power, the RF pulse can be readily scaled to the desired tip angle. If the initial angle is different from the desired angle, the process is repeated quickly.
Although the calibration of 90.degree. pulses are quick and accurate, calibration of 180.degree. pulses are very inaccurate. The signal-to-noise ratio for this pulse sequence with three 180.degree. pulses is very low. A theoretically perfect 180.degree. used in this pulse sequence would result in a zero signal-to-noise ratio and thus, could not be used for calibration.
Another technique for calibrating the RF pulse requires electronic measurements to be made on the coil in the absence of a magnetic resonance signal.
The present invention contemplates a new and improved RF calibration scheme which overcomes the above referenced problems and others.